This edge effect is known to be a source of errors in the Enzyme Linked Immuno Sorbent Assay (ELISA) when the latter is carried out using microtitration plates (Denmark and Chessum, Med. Lab. Sci. (1978), 35, 227). An erroneous test result is obtained which is to be seen in the fact that the color intensity in the edge wells of the microtitration plates used is increased, although a nearly identical extinction value in all wells was to be expected, based on the test arrangement employed.
This typical increase of the color intensity should not be confounded with individual deviations, the so-called outliers which seem to be distributed at random across the microtitration plate. This increase is caused by errors in the test performance, a nonhomogenous plate coating or a low quality of the plate material used.
The edge effect, on the contrary, is caused by a temperature gradient between the edge wells and the residual wells of the microtitration plate during the immuno-logic reaction and the enzyme reaction of the ELISA (Burt et al., J. Immunol. Meth. (1979) 31, 231).
In the case of a temperature rise by up to 1.6.degree. C. in the edge wells, temperature-dependent steps such as the antigen-antibody binding or an enzyme reaction proceed more quickly in said wells than in the residual wells of the plate. This is demonstrated by a greater color intensity of said wells in the ELISA.
The temperature gradient between the edge wells and those in the central part is caused by more rapid heating of the plate edge. This heating may occur when the plate is placed on a support having a good heat conduction, for example the metal bottom of an incubator as well as due to the heat insulation of the central part of the plate by the air cushion below. The higher the incubation temperature and the shorter the incubation times, the more pronounced is generally said edge effect. Said edge effect may be reduced by superposing the plates and can be eliminated by floating the plates bubble-free in a warm water bath or by using appropriate heating fans.
However, both of the latter possibilities are either difficult to perform or involve much expenditure from the technical point of view (Oliver et al., J. Immunol. Meth. (1981) 42, 195).